Field of the Invention
The present invention relates generally to fully bonded foam pre-insulated piping systems and, more specifically, to a method and apparatus for the continuous production of such piping systems.
Description of the Prior Art
Insulated pipelines are needed in a variety of situations. For example, distributed HVAC (heating, ventilation, and air conditioning) applications utilize chilled water for cooling and steam and hot water for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam and hot water are distributed to other locations. For example, on a school campus the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings. A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulted pipelines is used to carry the steam or hot water from the boiler to the other locations and back to the boiler. It is necessary for the pipes to be insulated in order to retain the internal temperature of the medium being transported and keep heating and cooling losses at a minimum. The insulated pipelines are usually located underground.
So called “pre-insulated piping systems” of the type under consideration are conventional and commercially available. There are predominately two types of such pre-insulated piping systems in use: Class-A drainable dryable testable (DDT); and polyurethane or polyisocyanurate “fully bonded” foam systems. In the bonded type system, the foam and outer jacket, being bonded, do not move relative to the inner pipe. In the Class-A type system, on the other hand, the insulated inner pipe is designed to move independently of the associated outer jacket. In fact, there is an air gap between the inner pipe and outer carrier pipe in the class-A type system.
The present application is directed toward the bonded foam type system. These systems utilize a steel pipe to convey fluid, i.e., steam and/or superheated water, where the fluid is at a different temperature as compared to the ambient environment. Around the outside of the steel pipe is a layer of insulating foam such as, for example, polyisocyanurate foam. In the case of high temperature piping systems, the insulating foam serves to keep heat loss from the starting location of the pipeline to the ending location at a minimum. Around the outside of the foam is a thin jacket of thermoplastic material, such as high density polyethylene (HDPE). The plastic jacket protects the foam from mechanical damage and also provides a watertight seal to prevent corrosion of the steel pipe. Although steel is commonly used fix the inner pipe which carries the media to be piped, copper, aluminum or other metals as well as fiberglass, PVC, and similar materials may be utilized, as well.
There have been a number of efforts in the prior art to produce “pre-insulated piping” of the above described type in continuous, rather than batch or one section of the pipe at a time processes. The simplest method of producing such pipe in continuous fashion would be to move the pipe forward in a production line, as on a conveyor, while it is simultaneously being rotated and while spraying the insulating foam onto the outer surface of the pipe. The foam would then typically be enclosed with an insulating jacket, all while being rotated and passed down the conveyor line. This approach is not possible, however, in the case where heat tubes are welded to the exterior of the pipe being insulated prior to the pipe being moved down the conveyor line. The presence of heat tubes on the exterior of the steel pipe makes it impossible to rotate the pipe while moving the pipe forward. As a result, it is necessary to develop other techniques for the continuous production of “pre-insulated” piping of the type under consideration.
U.S. Pat. No. 3,223,571 is an early continuous process. A film folding device is disclosed which provides a film covering spaced about the pipe into which the foam is injected. The pipe is continuously moved through the foaming apparatus and when it emerges from the apparatus, the foam is still generally soft or gel like. It is then allowed to set up and cure.
In the '571 process, as with other similar processes, a relatively complex molding apparatus is employed to accommodate the expansion and setting-up of the foam as the pipe is continuously moved through the molding apparatus. Many of these molding devices were designed similar to “corrugators” of the type used to manufacture corrugated plastic pipe. They featured relatively complicated and expensive endless complimentary moving die systems. Such a system is shown in FIG. 6 of U.S. Pat. No. 3,876,355. FIG. 3 of U.S. Pat. No. 7,824,595 shows another such complicated “corrugator-style” molding apparatus in which a series of endless belt mold sections form a semi-cylindrical recess for shaping the foam insulation as it passes through the apparatus. The molds are generally moveable at approximately the same rate as the pipe as it moves axially downstream through the mold apparatus.
While the above and similar processes may have succeeded in producing pre-insulated piping in continuous fashion, the molding apparatus section of the process was overly complex and expensive to produce. Also, since the apparatus could only produce one size (diameter) of pre-insulated pipe, it would be necessary to undertake the expense of manufacturing a number of different molding devices to accommodate the manufacture of pipe of various diameters.
Thus, there continues to be a need for improvements in the pre-insulated piping manufacturing processes, particularly in continuous pip production processes.